Piezoelectric resonators have been used for many years as resonant circuit elements in bandpass filter networks, such networks being commonly referred to as crystal filters. Most commonly, the piezoelectric materials employed are either crystalline quartz or piezoelectric ceramic, such as various lead zirconate-titanate materials. The resonators broadly comprise a blank of the piezoelectric material on which input and output electrodes are formed, for example by plating thin layers of an electrically conductive material. An input signal is applied to the input electrodes of the resonator which vibrates and the output signal is taken from the output electrodes, this output signal having been modified by the resonator. Quite commonly, one pair of electrodes serves both input and output functions. In such devices at frequencies above about 1 MHz the modes of vibration employed are almost invariably thickness modes such as thickness-shear or thickness-extension. Such resonators are generally characterized as bulk wave resonators.
In the theory of electric wave filters, it is well known that two or more resonators coupled in tandem fashion, with the first and last resonator being in addition coupled to, respectively, a source (or generator) and a load (or detector) constitute a bandpass filter, the bandwidth being determined by the degree of coupling. The use of electrical coupling between individual resonators or a plurality of resonators may require numerous interconnections and additional circuit elements including inductive devices such as balanced transformers. During recent years, elastically-coupled (acoustically coupled) thickness-shear resonators have been used to form multi-resonator bandpass filters which eliminate many of these circuit elements and connections. Such multi-resonator structures are commonly called monolithic crystal filters. They are described, for example, in an article in Electronics, vol. 45, No. 3, pages 48-51, dated Jan. 31, 1972, written by R. C. Smythe. In general, in structures of this type the coupling between adjacent resonators on a common piezoelectric plate occurs due to a trapping effect under the electrodes, the latter normally being of the plated type.
Monolithic crystal filters have found wide application, being simpler, smaller, and less costly than conventional crystal filters. However, serious limitations on the maximum bandwidth and center frequency which may be obtained in practice with monolithic crystal filters arise due to the fact that at high frequencies the piezoelectric plate constituting the main body of the filter becomes extremely thin and fragile. By using overtones of the fundamental thickness modes, a thicker plate may be used, but this in turn imposes severe limitations on the maximum bandwidth.
A variety of piezoelectric filter devices employing elastic surface waves recently have been devised for various signal processing applications at frequencies from roughly 30 MHz to above 1 gHz. Among these are a number of bandpass filter structures which achieve their response characteristic by frequency dependent interference effects rather than by use of resonance effects. For example, a pair of interdigital transducers is frequently used as a bandpass filter. In such structures the filter bandwidth is generally inversely proportional to the length of one or both transducers. This gives rise to a disadvantage in that in order to achieve narrow bandwidths the length of one or both transducers may be excessive. Moreover, special structures such as unidirectional transducers are required if the insertion loss of these filters is to be low. In this case the size and complexity of the structure are further increased.
More recently, elastic surface wave resonators have been described which functionally resemble bulk wave resonators such as the piezoelectric thickness-shear resonators previously mentioned. Because of their high Q and, for suitable materials, good temperature stability, such resonators may be used as circuit elements in bandpass filters in the same manner as individual bulk wave resonators were used prior to the development of the monolithic crystal filter. Such resonators were first described by E. A. Ash in a paper titled "Surface Wave Grating Reflectors and Resonators" presented at the 1970 International Microwave Symposium in Newport Beach, Calif., May 11-14, 1970, an abstract of which appears in the Digest of the IEEE Symposium on Microwave Theory and Techniques, 1970, p. 385. In this presentation Ash described the basic idea of using grating type reflecting structures to form high Q surface resonators, presented a partial theory of operation and gave preliminary experimental results. However, the work of Ash did not take into account the transverse mode structure of the resonator, recognition and control of which is of great importance in suppressing unwanted resonances in the resonator response characteristic. Further, Ash did not suggest methods and configurations for achieving elastic coupling between resonators either in the longitudinal direction of the resonator or in the transverse direction in order to construct a bandpass filter in which electrical coupling is unnecessary, or for some other purpose.
Following the work of Ash, C. S. Hartman and R. C. Rosenfield in U.S. Pat. No. 3,886,504 have described resonators which are essentially identical to Ash's and also the use of electrical coupling between two or more surface wave resonators in order to produce improved frequency selective characteristics. In addition, the patent teaches one form of elastic coupling in the longitudinal direction of the resonators. However, the patent does not recognize the existence of transverse modes and does not take into account the transverse mode structure in determining the response characteristic of a resonator or in its effect on elastic coupling between resonators. In addition the patent does not suggest any form of transverse elastic coupling between resonators.